Molecular Gas Disk in the Flocculent Spiral Galaxy NGC 4414

The central 3 x 6 kpc of the flocculent spiral galaxy NGC 4414 has been observed in the CO (J = 1-0) line with a 4" (190 pc) resolution using the Nobeyama Millimeter Array. Molecular gas is found to be distributed in a relatively smooth axisymmetric disk with a central hole of 0.5-1 kpc diameter. There is no prominent spiral or bar structure or noticeable streaming motions. Several clumps of molecular gas (called "disk GMAs,,) are found to be located randomly in the gas disk, with masses of a few 10^7^ M_sun_ and diameters of ~400 pc. The molecular gas distribution agrees well with the Hα distribution imaged by Pogge, including the clumps and the central hole. The correspondence between the Hα peaks and the disk GMAs indicates that disk GMAs are the main sites of massive star formation in this flocculent galaxy. Stability analysis of the gas disk using the Toomre Q parameter shows that the gas disk is unstable but close to neutral stability everywhere except the central hole. The gas disk has a nearly constant radial Q profile owing to the radial variations of the gas surface density and the gas velocity dispersion. Masses of the observed gas clumps agree well with the predicted masses of the most unstable mode. These results suggest that the disk GMAs were formed through gravitational instability of the gas disk. It is found that two disk GMAs have too large a velocity dispersion to be gravitationally bound. They are probably dispersing at present as a result of star formation. These observational results are consistent with the scenario that the molecular gas disk in this galaxy has been maintained around neutral stability by a feedback mechanism. The steps involved in the feed-back are clump formation through instability, star formation at the clumps, and clump disruption by star formation. The formation disruption cycle of a disk GMA takes several 10' yr in the center of NGC 4414.